The present invention relates to a rack and pinion steering apparatus, which is used extensively as a steering apparatus for vehicles, and a method of manufacturing a rack shaft of a rack and pinion steering apparatus.
A rack and pinion steering apparatus comprises a pinion shaft, which is connected to a steering member such as a steering wheel and is provided with pinion teeth formed on the surface thereof, and a rack shaft, which extends from the right and left sides of a vehicle body and is provided with rack teeth juxtaposed on the outer surface of a middle portion thereof over a proper length. The rack and pinion steering apparatus is constructed to transform the rotation of the pinion shaft in accordance with the manipulation of the steering member by the driver into the axial movement of the rack shaft and to activate wheels for steering (right and left front wheels, in general) connected to the right and left ends of the rack shaft via respective steering tie rods.
In such a rack and pinion steering apparatus, the pinion teeth on the pinion shaft and the rack teeth on the rack shaft are generally formed through a cutting process using a metal cutting machine tool such as a gear cutting machine or a broaching machine in order to obtain high mesh accuracy, and the tooth flank often undergoes a heat treatment in order to increase the surface hardness and the durability (see, for example, Japanese Patent Application Laid Open No. H06-264992 and Japanese Patent Application Publication No. H08-25462).
In manufacturing of the rack shaft of the rack and pinion steering apparatus mentioned above, the rack shaft conventionally undergoes a gear cutting and then a heat treatment such as the induction quenching and tempering for the purpose of surface hardening. In the heat treatment, there is a problem that a straightening process for eliminating distortion, which may occur at the tooth flank or the shaft portions, becomes a major obstacle to the manufacturing processes. Moreover, there is another problem that the quenching may cause defects such as a quench crack.